I think he wants a 1337 converter to work when he is typig rather than when the message is sent. What you want to do in the area that you are unsure of is use the following code.
code:
ChatWnd.EditText_ReplaceSel("4");//Insert 4 where the caret is (the blinking thig deoting where you are typing)
return true;//Do not let messenger put in the a
You also may want to look at a switch statement rather than many if statements for each key, it will just turn out nicer if you understand how to use them. This isn't needed but it is an ideal opportunity to give it a go and learn how to do it.